The invention relates to the field of tubular motors of the induction motor type (or asynchronous motor) with a squirrel cage rotor and two directions of rotation used in tubular actuators designed for an installation in a building.
Tubular motor means a motor inserted into an outer tube containing the tubular actuator and of which the length of the stack of stator metal laminations is usually greater than its outer diameter. The tubular actuators are installed in a horizontal position in a building, for example for winding fabrics of awnings or screens or for winding roller blinds.
The tubular motor is therefore one of the components contained in the tube of the tubular actuator. The other components are a reduction gear and usually a control unit. The result of this is that the vibrations of the motor are transmitted directly or indirectly to a long tube, of which at worst they can excite normal modes and/or of which at least they use all of the surface as a radiative surface. Particular attention must be paid to limiting or attenuating these vibrations, all the more so since the use of these actuators is mainly associated with comfort in the dwelling or with managing the luminosity or solar gains in offices, and operating silence is required in these applications.
Because the motor is simultaneously a tubular motor and a horizontal-axis motor causes problems of controlling the noise level, which are also aggravated by the specific features of an induction motor. These problems relate mainly to the various causes of eccentricity of the rotor in the stator bore. The eccentricity is all the more difficult to remove if the rotor is long and/or if the guidance of the rotor is not provided as near as possible to the stator.
In particular, a dual-pole induction motor powered at 50 Hz has a field rotating at 50 rps. Relative to the rotor, this field drives a 100 Hz pulsating force the amplitude of which is modulated to twice the sliding frequency of the rotor relative to the rotating field. This modulation also applies to all of the pulsating frequencies and particularly to the pulsations due to the passing of notches the frequency of which is equal to the product of the frequency of rotor rotation by the number of stator notches (namely close to 600 Hz for a motor with 12 stator notches rotating, with no load, at close to 50 rps).
When the motor with no load rotates for example at 48 rps, the frequency of modulation is 4 Hz. The result of this is a very low-frequency modulated noise, like a drumming, which is both disagreeable and greatly harms the perceived quality of the motor.
Economically, a shaft is guided by plain bearings (or solid bearings) rather than rolling bearings. As a result, there is a slight radial clearance of the shaft in the bearing, which aggravates the above phenomenon when it operates in the same direction as gravity. The intensity of the very low-frequency modulated noise then depends on the orientation of the motor about its horizontal axis, which also harms the perceived quality. Because the motor has two directions of rotation it requires the presence of two or three phases in order to be able to reverse the direction of the rotating field. These phases are produced with the aid of windings inserted into the stack of stator metal laminations. As a result, each end of the motor is completely taken up by the winding head, also called end winding. Each winding head is partly or most frequently completely covered by an insulating capping, designated by insulation casing or bowl. The presence of these winding heads usually separates the rotor bearings from one another. The smaller the diameter of the tubular motor, the proportionally longer the leading-out wire. Therefore, the tubular motors to which the invention relates have bowls with a length that is usually at least greater than the radius of the motor.
Patent EP 0410933 describes an induction motor with a squirrel cage rotor and of the tubular type used in the field of the invention. The rotor shaft is guided by rolling bearings at a distance from the rotor and held by end-plates centred by the outer tube.
The induction motor is also characterized by a narrow air gap. A typical value is 0.4 mm for a tubular motor with a diameter of between 40 and 60 mm. The value of this air gap is limited only by the clearance necessary for the free rotation of the rotor in the stator bore taking account of the possible geometric faults, the latter being aggravated by the considerable distance between the bearings. This value normally renders inapplicable centring solutions that can be envisaged for other types of motor, and in particular motors of which the rotor comprises permanent magnets and supporting much greater air gaps.
U.S. Pat. No. 3,873,861 describes a non-tubular, squirrel cage motor, the rotor of which is supported by two plain bearings. These bearings are themselves inserted into end-plates. One of the end-plates consists of a first casing for insulating the stator windings. The other end-plate is connected to the second casing for insulating the stator windings. The bearings are relatively close to the rotor, but the centring of the rotor shaft is carried out by means of the insulation casings. Since the motor is short, the problems mentioned above are probably not significant.
U.S. Pat. No. 5,166,565 describes a similar principle of guidance by means of the casings for insulating the stator windings, this time performed by rolling bearings.
In the case of the motor described in U.S. Pat. No. 6,515,385, guidance is provided by rolling bearings very far from the squirrel cage rotor.
By contrast, U.S. Pat. No. 6,399,007 describes a non-tubular, hidden-pole induction-motor structure with a short rotor, for which the rotor is guided by plain bearings that can be kept very close to the rotor (FIG. 6). This configuration is made possible by the structure of the winding (not shown) around the inner portion of the stator in these particular motors with a single direction of rotation. The bearings have a much smaller diameter than the stator diameter.
With the exception of U.S. Pat. No. 6,399,007, which relates to a non-reversible motor type, all the above motors have bearings or rolling bearings held by supports that are mainly connected to the outer tube or mainly connected to the electrical insulation means of the windings. On the matter of vibration, the radial micro-movements of the bearings are, in a first case, directly transmitted to the outer tube. In the second case, these micro-movements are constrained little or not at all and can have a significant amplitude.
There is therefore, for tubular actuators of buildings, a need to produce a low-cost tubular motor of the induction type with a squirrel cage rotor making it possible to ensure good accuracy of alignment of the rotor and the stator and a very low excitation of the actuator tube by the vibrations transmitted to the bearings supporting the rotor.
U.S. Pat. No. 5,945,759 and U.S. Pat. No. 2,462,204 disclose a motor including a stator comprising metal laminations forming a central bore and a support part of a bearing centred in the central bore.